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Chemistry investigation

HI all
I have recently completed the practical section of my A2 chemistry investigation, investigating reaction kinematics.

The reaction used was an iodine clock reaction between potassium iodide, potassium peroxodisulphate and sodium thiosulphate. The expected order of reaction is 1 for both KI and K2S2O8.

However my results show that the order is 2 (or 1.5 from predicted trendlines on excel) with respect to KI. All other aspects of the investigation went fine and returned expected outcomes.

I performed the investigation at 50 deg C, using 5 dilutions of KI. i repeated the experiment 6 times for each dilution and calculated averages disregarding anomalies. When I discovered the graph was not what was expected I repeated the entire experiment, so leaving me with 12 repeats for each dilution. I plotted graphs of true rate against true concentration, then checked them with graphs of true rate against true concentration squared. All the evidence supports an order of more than 1.

The concentration of KI was 1mol diltued with water to make 5ml solutions (i.e 1ml KI 4ml water, 2ml KI 3ml water etc). The amount of K2S2O8 was 2ml of 0.025mol, thiosulphate was 2ml of 0.01mol, and finally 2ml of starch (as an indicator).

I have no idea why the experiment returned the results it did (neither do my teachers) so any help would be greatly appreciated. Possible theories are breakdown of reactants at higher temperatures and systematic errors (although hard to believe with 6 repeats and 2 independant tests)

Reply 1

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nobrainer

I have no idea why the experiment returned the results it did (neither do my teachers) so any help would be greatly appreciated. Possible theories are breakdown of reactants at higher temperatures and systematic errors (although hard to believe with 6 repeats and 2 independant tests)

Figures aside (I don't really remember that much Chem), wouldn't the fact that all the repeats showed the same error imply that it was a systematic problem? Could something have contaminated the reactants?

Reply 2

nobrainer

i thought it may be a systematic error, carried out for all results. however that would still give a straight line although offset from the origin.... my graph is a distinct curve which would imply the concentrations are changing throughout the experiment which is not good :s-smilie: